1. Field of the Invention
This invention relates to fabrication of semiconductor devices arid, more specifically, to the fabrication of gate structures for MOSFET semiconductor devices and particularly PMOS semiconductor devices.
2. Brief Description of the Prior Art
Standard MOSFET semiconductor devices generally include a gate electrode separated from a channel by a dielectric or gate oxide, generally silicon dioxide. PMOS devices generally have a highly doped P-type gate electrode over an N-type doped substrate (also known as an N-well) to provide a P-channel, and NMOS devices generally have a highly doped N-type gate electrode over a P-type doped substrate (also known as a P-well) to provide an N-channel. The gate electrode is generally polysilicon and the dopant generally used for PMOS devices is boron. In the present state of the art, the most aggressive gate oxide can be as thin as about 15 Angstroms and is generally about 45 Angstroms, though it can be even thicker, the thickness of the gate oxide being dependent upon device geometry which is continually being reduced in size. The fabrication process generally involves formation of the gate oxide with formation of a patterned gate electrode over the gate oxide.
In the case of PMOS devices with boron-doped polysilicon gate electrodes, the boron has a tendency to diffuse to some extent through the gate oxide and into the channel. This boron doping in the channel causes the threshold voltage of the device to shift, this shift being undesirable. Accordingly, it is highly beneficial to provide a barrier to the diffusion of the boron into the channel. Traditional prior art approaches to this problem have been, for example, ROXNOX, which stands for reoxidized ammonia nitrided oxide, wherein the gate dielectric processing, involves application of ammonia at high temperature to the gate oxide to introduce nitrogen as a barrier to the undesirable boron diffusion. While it is a thermal process, it introduces nitrogen in high concentrations at the bottom of the gate oxide that increases fixed charge and interface-state density which degrades device performance and is therefore undesirable. A second procedure is known as N2O or NO thermal oxidation wherein nitrous oxide or nitric oxide is introduced to the gate oxide at elevated temperature with the nitrogen concentration being directly proportional to the increase in temperature. Again, this procedure is a thermal process and places nitrogen predominantly at the bottom portion of the gate oxide and in the region of the interface between the gate oxide and the channel, which degrades device performance when present in large concentrations.
In the case of CMOS devices with metal gate electrodes (e.g., W/TiN stack), interaction between the metal gate and the ultrathin oxide can pose a formidable challenge. Accordingly, it is highly beneficial to provide a nitride or oxynitride barrier to prevent detrimental metal-oxide interactions (e.g., spiking) at the top gate electrode-oxide interface.
In accordance with the present invention, the above identified problem of the prior art is minimized.
It has been determined that the undesirable properties resulting from the inclusion of nitrogen in the gate oxide results from the fact that the nitrogen is disposed in the gate oxide, but in the region of the gate oxide immediately adjacent or at the interface between the gate oxide arid the channel. Accordingly, in accordance with the present invention, the nitrogen is disposed preferably in the portion of the gate oxide remote from the channel or as a separate layer at the surface of the gate oxide remote from the channel, this barrier being at least one or two monolayers or from about 5 to about 10 Angstroms and generally as thin as possible commensurate with its purpose as a barrier to diffusion of boron into the channel. The nitride and/or oxynitride layer is formed prior to introduction of the dopant either into the gate electrode material after formation of that electrode, such as by implantation, or concurrently with the gate electrode formation, such as by CVD. An important feature of the invention is that the nitrogen species utilized in accordance with the present invention is an uncharged or neutral beam species, since it has been determined that charged particles in a plasma can potentially cause charge-induced damage which leads to degradation in gate dielectric integrity, this problem becoming more acute as device dimensions are reduced. In other words, the nitrogen introduced to the oxide surface is by chemical reaction with uncharged or neutral atomic nitrogen.
According to a first embodiment of the invention, the gate oxide of the CMOS device is initially formed by conventional thermal oxidation in oxygen, nitrous oxide, nitric oxide or any similar oxidizing gas. In the case of the nitrogen-containing oxidizing gas, a very small amount of nitrogen will be incorporated at the gate oxide/silicon substrate interface as well as elsewhere through the gate oxide, however the amount of nitrogen will be insufficient to cause any of the problems encountered in the prior art as discussed above to any significant extent while providing some of the benefits of increased gate dielectric reliability having nitrogen at the interface of the gate oxide and substrate. After the gate oxide has been formed in this manner, the gate oxide upper surface remote from the substrate is subjected to nitridation using a neutral N-beam at a temperature preferably in the range from about standard room temperature to about 300 degrees C. The neutral N-beam is produced by well known techniques, such as thermal processes, nozzles, laser ablation, adiabatic expansion and plasma techniques ill conjunction with electrostatic grid neutralizers and plate neutralizers.
According to a second embodiment, the surface of the silicon substrate is nitrided using a neutral N-beam. The neutral N-beam is produced in the same manner discussed above in connection with the first embodiment. The device is then subjected to thermal oxidation in oxygen, nitrous oxide, nitric oxide or any similar oxidizing gas, during which the nitrogen is displaced from the oxide/silicon interface and moved into the bulk of the oxide.
The CMOS device fabrication is then completed in standard manner.
In accordance with the present invention, there is controlled nitrogen incorporation at the gate oxide surface remote from the substrate. In addition, there is a low thermal budget and there is little charge-induced damage which is generally associated with plasma-based processes. Furthermore, ammonia, the hydrogen of which has been shown to be detrimental, is not used for nitrogen incorporation as in prior art procedures.
In addition, plasma processes are generally followed by an annealing step. With the use of the neutral nitrogen atom species, the annealing step is not required. Accordingly, tile additional annealing step of the prior art is eliminated.